Repeater circuits



1,689,318 B. G. BJRNsoN REPEATER CIRCUITS Oct. 30, 1928.

Filed June 6, 1927 F- T -lllll'lP- QIIIIII 'Arran/Wgr Patented Oct. 30,19.28.

UNITED STATES' :BJRN G. RJRNSON, OE NEW YORK, N. Y.,

ASSIGNOB T0 BELL TELEPHONE LABORA- TORIES, INCORPORATED, or NEW YORK, N.Y., A CORPORATION OE NEW YORK.

REPEATEB CIRCUITS.

Application filed June 6,

This invention relates to transmission systems and particularly tosignal controlled circuits and is in part a continuation of applicationSerial N o. 142,566, filed October 19, 1926. f

An object of the invention is to reduce the edect of line noise on theoperation of signal controlled circuits. I

One of the chief sources of trouble encoun tei'ed in connection with theoperation of signal controlled circuits such as are used for voiceoperated repeaters, echo suppressors and the like is the so-called linenoise. Voice controlled circuits have been devised which operate verysatisfactorily on quiet lines but are either commercially inadequate orafford poor quality on noisy lines. Heretofore this difficult-y has beenOvercome to some extent by the use of circuit arrangements adapted todifferentiate between a steady current and a sudden impulse. Theoperation of such arrangements is based on the fact that speech energyis transmitted in impulses of short duration corresponding to spokenVwords, while noise energy in effect is in gen eral relatively constant.

In one embodiment of the invention, which is disclosed also but notspecifically claimed in Fig. 4 of applicants prior application referredto above, this differentiation between noise and speech is obtained bycoupling branched paths of the line over which the signaling currentsarev transmitted and subject to noise currents, through resistancecouplings to the input circuit of the relay controlling vacuum tube,each of the branched paths containing a vacuum tube detector and onecontaining in addition a delay circuit.

The detector tubes in the branched paths are relatively adjusted so thatthe steady noise currents below a certain level transmitted thereoverare continuously neutralized as,to their effect on the relay controllingtube,

. while the delay circuit serves to delay the speech impulses in onebranch until the relay controlling tube is operated by the speech1mpulse transmitted through the other branch.

In a' specific, preferred, embodiment, the invention comprises a mainand an auxiliary path, each containing an electric discharge detector orrectifier the input circuits of which are connected in parallel to asignaling line subject to noise. The output of the main path is coupledthrough a resistance to the 1927. Serial No. 196,763.

output of the auxiliary path is coupled to the input circuit of theamplifier through a second resistance so as to oppose the action of themain path. The auxiliary path contains a delay circuit which acts todelay the passage therethrough of signaling impulses which are of shortdui-ation While any comparatively steady currents, such as noisecurrents, below a certain energy level determined by the saturationpoint of thedetector in the auxiliary Epath will neutralize the effectof similar currents in the output circuit of the main pat-h on theamplifier. The output of the final detector is connected to a relay tobe controlled by the signal currents. The circuit is so adjusted that,for impressed voltages below a certain value which is determined by themaximum permissible noise level, the output currents of the main and a.'liary detectors annul each other intheir effects on the amplifier andfinal detector. Therefore, the" relay will not be affected by steadynoise waves of voltages below this value but will be sensitive to speechimpulses because the portion thereof transmitted `through the auxiliary`detector circuit are delayed therein and, therefore7 produce noneutralizing effect on the portion transmitted through the main detectorpath in the input circuit oit the amplifier. In rsome cases it may bedesirable to employ an amplifier in place of the final detector. By thearrangement described above the effect of the noise on'the line isneutralized by balancing rectified noise voltages at a pointin thesystem where the noise is least amplified. This permits greateraccurangement of Fig. l; and Fig. 3 shows graph,

- three-electrode electric discharge devices 7 ically the relation ofthe currents in the various portions of the circuitI of Fig. 2.

Referring first to Fig. 1, there is shown a circuit arrangement foropening a short-circuit across a line 5 in response to .wavestransmitted over a line 6. Such an arrangement may be used, for example,in connection with a voice .operated repeater or an echo suppressor. Inthe latter case it would usually be desirable to arrange the relay toclose a short-circuit across one line, in response to Waves in theother.

The circuit arrangement comprises two and 8 having their input circuitsconnected `in push-pull relation to the line 6 through an inputtransformer 9. The output of the device 8 is connected directly to aresistance 10 which is connected in series with a second resistance 11to the input electrodes of another three-electrode electric dischargedevice 12. A condenser is connected in shunt to the resistance 10 tosmooth out the rectitied wave. 'Ihe resistance 11 is connected to theoutput of the device 7 through a delay circuit 13 which comprises anordinary low pass filter consisting of series inductance and shuntcapacity. The output of the discharge device 12 is connected to thewinding of a relay 14,*the armature of which is arranged to normallyshort-circuit the line 5.

As explained above, ordinary noise currents in the line 6 arecomparatively constant and the two devices 7 and 8 are so arranged thatconstant voltages, below a certain lim iting value determined by thepermissible maximum noise level, produce substantially e ual outputcurrents in the two devices.

e circuit is so designed that for these voltages the potential dropsacross the equal resistances 10 and 11 will be substantially equal andcounteract each other in theireffeet on the amplifier 12. However,speech jwaves, which are of relatively short duration,

impressed on the devices 7 and 8 from the line 6 will produce differenteffects upon the amplifier 12, since those transmitted by the device y8will immediately affect the amplifier while those transmitted by thedevice 7 will be delayed by the delay circuit 13, and their eect on theamplifier 12 will, therefore, be delayed with respect to that yof thespeech waves transmitted by the device 8. These rectified speech wavesfrom the device 8 after being amplified by the device 12 will operatethe relay 14 to open the short-circuit across the line 5.

Fig. 2 shows a modification of the circuit arrangement of Fig. 1. Inthis figure waves from the line 6 pass through an input transformer 18and are impressed on the input of a three-electrode electric dischargedevice 17 which is arran ed to operate as an amplifier. Connected 1nparallel to the output electrodes ofthe device 17 are the primarywindings of the transformers 19 and 20. 'Ihe secondary winding of thetransformer 2O is connected to the input of a three-electrode electricdischarge device 21 which serves as a main detector. The secondaryWinding of the transformer 19 is connected to the input of athree-electrode electric discharge device 22 which serves as anauxiliary detector.

v'Ihe output of the detector -21 is coupled through a resistance 23 tothe input of a threeelectrode electric discharge device 24, which isarranged to operate as an amplifier. A condenser is connected in shuntto the output of detector 21 to smooth out the rectified Wave. 'lheoutput of the auxiliary detector 22 is connected through a delay circuit25 to the resistance 26, which is also connected in the input circuit ofthe ampliiier 24. Resistances 23 and 26 are so arranged that the voltagedrops produced across them by the output currents of the detectors 21and 22, respectively, oppose each other in their action on the amplifier24. The output of the amplilier 24 is coupled through a resistance 27 tothe input of a three-electrode electric discharge device 28. This deviceis preferably operated as a detector, but in some cases it may be founddesirable to decrease the grid bias` ing potential and operateit as anamplifier. The output of the device 28 is connected to the winding of arelay 29, the armature of which is connected to normally short-circuitthe line 5. The discharge devices are supplied with filament heatingcurretand space current 'through retardation coils in the usual manner.

In some cases it may be desirable to use a filter network in place ofthe condenser 30 as otherwise the highly ltered output of the detector22 will not be effective to neutralize the less thoroughly ltered outputof the detector 21. It is'desirable that such a network should have thesame attenuation characteristic as the delay network but produce noretardation eect. Such a circuit may be not practically realizable,however',

and a single section low pass filter will give good protection fromfalse relay operation though it will slow up the operation slightly.

` This circuit operates in a manner similar to the circuit of Fig. .1.For noise currents below a certain level, determined by the maximumpermissible noise level, the effects of the detectors 21 and 22neutralize each other on the input of the amplifier 24. Whereasv voicewaves which are substantially instantaneously transmitted b the pathincluding detector 21, while t e voice'waves transmitted over thekauxiliaryrpath are being delayed by the delay circuit 25 in the outputof the detector 22, are amplified by Ythe device 24 to operate the relay29.

The detailed o eration of the circuit can best be explaine withreference to Fig. 3

ofthe amplifier 17 to currents in various portions of the circuit. CurveA represents the current in the output of the main detector 21.

Curve B represents the current in the output of the auxiliary detector22. As indicated, the circuit is so adjusted that these currents are ofsubstantially the same magnitude up to the point of maximum noiseprotection, while beyond that point the output current of the maindetector 21 exceeds that of the auxiliary detector 22. This desiredcurrent characteristic is obtained by the use of a gridleal; resistance31 connected in the input circuit of the auxiliary detector 22. Curve Crepresents the output current of the amplifier 2li. As indicated, thisis practically constant up to 0.9 volt input, while above this point itdrops ofi very rapidly. D represents the output current of the detector28, i. e. the current supplied to the relay winding. As indicated, whenthe. output current of the amplifier 24 i begins to drop off, thiscurrent rises almost immediately to its final value.

Since speech impulses require a certain definite time to build up, thebeginning of a speech wave may be weaker than the lline noise eventhough the average speech energy is of a much higher level. However, ifthe speech impulse builds up to a level higher than the maximum noiselevel in a timeshorter than the time of transmission through the delaynetwork, the relay will be kept operated by the speech.

lVhat is claimed is:

l. In a signaling system, a path for wave impulses of' short durationand for relatively constant waves, branch paths connected thereto, arectifier in each of said branches, an amplifier, a connection from eachof said paths to the input of said amplifier, and means in one of saidbranches whereby for constant waves the effects of said branches on saidamplifier substantially annul each other, while for the wave impulsesthe effect of one ot' said branches predominates.

2. The combination according to the preceding claim in which said meanscomprises a delay circuit. I

3. In a signaling system, a path for wave impulses of short duration andfor relatively constant waves, main and auxiliary branch paths connectedthereto, a rectifier in each of said branches, a resistance element, apair -of electric discharge devices having input and output circuits andcoupled in tandem by means of said resistance element, connections fromthe output of each of said rectifiers to the input circuit of the firstof said devices,

means associated with the rectifier in said auxiliary path whereby forconstant waves the effects produced on said discharge devices by therectified currentsin said branchessubstantially annul each other, whilefor wave impulses the effect of the rectified current in the main pathpredominates, and a relay to be controlled by the wave impulsesconnected in the output circuit of the second of said devices.

4. A combination according to the next preceding claim in which theeffects of the currents in the main and auxiliary paths annui each otheronly for constant waves below a definite energy level while above thatlevel the effect otl the main path predominates..

5. In combination a signaling line adapted to transmit signal impulsesof short duration and subject to relatively constant noise Waves, a mainand an auxiliary rectifier each having its input associated with saidline, an electric discharge device having input and output1 circuits, animpedance element common to the input circuit of said device and theoutput of said main rectifier, a second impedance element conunon to theinput circuit of Said' device andthe output of said auxiliary rectifier,means associated with said auxiliary rectifier whereby the voltage dropsproduced in said impedances by the noise waves substantially balanceeach other in their effects on the discharge device while for the signalimpulses the effect of the voltage drop in said first impedance elementpredominates, and a relay to be controlled by said signaling impulsesconnected to the output circuit of said device.

6. A combination according to the next preceding claim, and meansassociated with said. auxiliary rectifier whereby the voltage dropsproduced in the impedance elements by the noise waves only annul eachother for waves below a definite energy level while above that level thevolt-age drop in the impedance associated with the main rectifierpredominates.

In witness whereof, I -hereunto subscribe my name this 81st day of May,A. D., 1927.

BJRN G. BJ RNSON

